1. Field of the Invention
This invention relates to a method for recording a digitized audio signal and the like on a recording medium loaded in a recording and reproducing apparatus of rotary head type and also relates to such a recording and reproducing apparatus.
2. Description of the Prior Art
With the progress of the LSI technology in recent years, digital processing of both audio signals and video signals has been greatly advanced. Audio visual apparatuses capable of recording and reproducing an audio signal and a video signal in a digitized form are now widely being put into practical use. Also, digital VTR's for both commercial and consumer use, are being widely used which record an audio signal and a video signal in a digital signal form. This is advantageous in that no degradation of the reproduction signal quality of a copy of the recording theoretically occurs. These digital recording apparatuses are designed so that, even when an error attributable to dropouts distributed on a recording medium occurs, such an error is corrected by the function of an error detection code which is added to recorded signal data to generate a reproduced output signal having the high quality equivalent to that of the analog input signal recorded on the recording medium after the A-D conversion.
In the case of the digital VTR described above, it is necessary that, for the purpose of high-fidelity reproduction of the original analog input signal, the signal is to be sampled at a sampling frequency two or more times as high as the frequency band of the original analog signal, and the number of quantized bits is to be selected to be as many as possible. Therefore, an extremely wide frequency band is required in the case of the digital VTR as compared to a conventional analog VTR of rotary head type, and signal recording with a high recording density is essentially required.
For the attainment of the desired high density recording, it is naturally required to shorten the recording wavelength and to narrow the width of the recording track. However, accumulation of highly advanced recording manufacturing technologies is needed for the attainment of the above requirements, and the desired high density recording with a density several times as high as the conventional density cannot be so easily attained.
Thus, a common practice now employed for the realization of the digital VTR is the multichannel recording of a signal by a plurality of heads, and the so-called segmented recording in which the rotary drum carrying the recording heads is driven at a high rotation speed so as to divisionally record the signal on a plurality of segmented regions of a recording medium.
On the other hand, the number of channels of an audio signal recorded on the recording medium in the digital VTR tends to be increased to four or more channels from the conventional two stereo channels. Especially, in the case of the digital VTR used for commercial service such as broadcasting service, the number of channels is preferably as many as possible, because it is convenient for the purpose of, for example, later editing.
The so-called D.sub.1 and D.sub.2 formats standardized by the SMPTE (The Society of Motion Picture and Television Engineers) are employed in broadcasting digital VTR's now in practical use. In these digital VTR's, four audio recording channels and both the multichannel recording technique and the segmented recording technique described above are used. That is, the recording region corresponding to one field period is divided into a plurality of segments. Further, in the case of the D.sub.1 format, audio signals are recorded on areas at the center portion of a magnetic tape in a relation separated from video signals, while in the case of the D.sub.2 format, audio signals are recorded on areas at both sides of a magnetic tape in a relation separated from a video signal. That is, according to each of these formats, audio recording regions in the form of independent sectors classified by the individual unit audio channels are provided on the tape so as to enable editing of audio information belonging to each audio channel, and a time-base compressed audio signal is recorded on the audio recording regions.
The problem encountered with the above manner of digital audio signal recording is the possible occurrence of burst-like dropouts attributable to dust particles, scars and the like that may be two-dimensionally distributed on the recording medium. In order to deal with such a problem, it is necessary to suitably arrange audio data and an error correction code so that resultant errors can be dispersed as much as possible and converted into correctable random errors. It is also necessary to allocate the individual audio channels to the recording sectors so that each of the audio channels has an equivalent and maximum error correction capability to withstand such an error.
A prior art example of such a system intended for attaining multichannel segmented recording of an audio signal by allocating a plurality of audio channels to a plurality of recording sectors is described in JP-A-61-160803 entitled "Digital Signal Recording System."
The cited patent application discloses a manner of arranging, on a tape, a plurality of recording sectors to which respective audio channels are allocated. In the disclosure of the citation, the relation between a specific head and an associated audio channel is considered so as to minimize an adverse effect attributable to possible clogging of the gap of the specific head.
FIG. 6 shows a second embodiment described and illustrated in the Japanese patent application cited above. In FIG. 6, the reference numeral 41 designates video signal recording sectors on a magnetic tape, and 42 and 43 designate audio signal recording sectors arranged on both sides respectively of each video signal recording track sector 41 at positions adjacent to the respective marginal edges of the tape. The hatching A.sub.1 indicates each of the recording sectors of the first audio channel. The symbols A and B are used to distinguish the recording tracks formed by two-channel head pairs angularly spaced apart on a rotary drum by 180.degree..
The cited patent application describes that, by employment of the arrangement shown in FIG. 6, multichannel recording can be attained by the two-channel head pairs mounted on the rotary drum in the 180.degree. spaced apart relation. It also describes that, by increasing the rotation speed of the rotary drum to a level five times as high as that used for conventional analog recording, four audio channels can be recorded on ten tracks per field. One of the audio channels shown in FIG. 6 will now be noted. It will be seen in FIG. 6 that the first audio channel is allocated to one of the two recording sectors so as to avoid the signal recording (reproduction) by the same single head only and so that the individual heads can uniformly participate in the signal recording (reproduction). It will also be seen in FIG. 6 that the recording sectors belonging to the first audio channel are spaced apart by a largest possible distance in the longitudinal direction of the tape so as to alleviate the adverse effect of a burst-like dropout.
Therefore, although occurrence of clogging of the gap of any one of the four heads will lead to the loss of 1/5 or 2/5 of information to be recorded on the individual tracks of the audio channels when the unit of the signal format is one field, audio information free from the adverse effect of dropouts can be reproduced by means of signal recovery according to error correction or by means of linear interpolation average of the preceding and succeeding sampled values on the basis of data recorded on the other sectors which are not adversely affected by the head clogging.
On the other hand, the television broadcasting system is broadly classified into the NTSC system and the PAL system. These two broadcasting systems are different in the required frequency band and the manner of chrominance signal processing due to the difference in the number of scanning lines and the field frequency. Therefore, an analog VTR for use for the NTSC system differs inevitably from that used for the PAL system. However, in the case of a digital VTR, there is not any great difference between that used for the NTSC system and that used for the PAL system in the way of signal processing except for the difference in the data recording capacity (the data transfer rate) in the stage of signal processing after digitizing. Therefore, a single digital VTR can be used for recording both the NTSC signal and the PAL signal by switching so as to deal with recording of either the NTSC signal or the PAL signal. In this case, in view of the frequency band and the field frequency, the NTSC system drives the rotary drum at the rotation speed of three times of 1,800 rpm of the conventional device, and the PAL system drives the rotary drum at the rotation speed of four times of 1,500 rpm of the conventional device, so as to achieve the signal recording by the two-channel four heads.
However, although the Japanese patent application cited above describes the basic idea with respect to the allocation of a plurality of audio channels to audio recording sectors according to the signal format in which one field is its unit, the citation does not clearly describe any concrete and universal rule which sets forth the optimum relation between the audio channels and the audio recording sectors. Also, the Japanese patent application cited above does not clearly describe a more concrete sector arrangement for the audio channels in which the aforementioned difference between the NTSC system and the PAL system is taken into consideration.